1. Field of the Invention
The present invention relates to a gas sensor including a detection element which is exposed to a gas-to-be-detected in order to detect a particular gas component contained in the gas-to-be-detected.
2. Description of the Related Art
A gas sensor has conventionally been known which is attached to an exhaust pipe of an automobile or the like. The gas sensor includes a detection element which generates an electromotive force whose magnitude changes in accordance with the concentration of a particular gas, such as NOx (nitrogen oxides) or oxygen, contained in a gas-to-be-detected (exhaust gas), or which changes as the resistance of the detection element changes in accordance with the concentration of the particular gas. Patent Documents 1 and 2 disclose such a gas sensor which includes a detection element extending from a base end side toward a forward end side in an axial direction thereof; a tubular metallic shell which surrounds the circumference of the detection element; and a tubular protector (inner protector) provided on the forward end side of the metallic shell and surrounding the circumference of a forward end portion of the detection element.
The forward end portion of the detection element has a detection section which has a gas passage portion which allows the gas-to-be-detected to pass therethrough. The above-described gas sensor detects a particular gas component contained in the gas-to-be-detected which is introduced into the interior of the detection element through the gas passage portion of the detection section. The protector (the inner protector) has gas introduction holes which are located on the base end side with respect to the forward end of the detection element and which introduces the gas-to-be-detected from the outside of the protector into the interior of the protector, and a gas discharge hole which is located on the forward end side with respect to the forward end of the detection element and which discharges the gas-to-be-detected from the interior of the protector to the outside of the protector. Notably, the forward end portion of the detection element projects forward from the metallic shell such that the entire gas passage portion is located on the forward end side with respect to the forward end of the metallic shell.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2009-97868
[Patent Document 2] DE 10 2008 041 038 A1
3. Problems to be Solved by the Invention
In recent years, there has been demand to decrease the length of the detection element so as to reduce cost. This is because the amount of platinum used to form electrodes, lead portions, etc., on the detection element can be reduced by decreasing the length of the detection element. In the case where the length of the detection element is decreased in the gas sensor disclosed in Patent Document 1, a forward end portion (a portion where a gas passage portion is provided) of the detection element is preferably located closer to the base end side (the side toward the metallic shell), while a rear end portion (a portion where connection pads are provided) of the detection element is arranged as far as possible from the forward end of the metallic shell. This is because, if connection terminals connected to the connection pads provided on the base end portion of the detection element are too close to the forward end of the gas sensor, the connection terminals deteriorate due to the influence of heat from the exhaust pipe, and the reliability of electrical continuity between the connection pads and the connection terminals is more likely to decrease.
In order to overcome such a drawback, in the gas sensor disclosed in Patent Document 1, a portion of the detection element located on the forward end side thereof is shortened such that at least a portion of the detection section (the gas passage portion) of the detection element is disposed on the base end side with respect to the forward end of the metallic shell. In this case, the detection section (the gas passage portion) of the detection element is disposed on the base end side with respect to the gas introduction holes of the inner protector. Incidentally, the gas-to-be-detected having been introduced into the interior of the inner protector through the gas introduction holes of the inner protector flows toward the forward end side, and is discharged to the outside through the gas discharge hole provided on the forward end side of the inner protector. Therefore, if the gas sensor of Patent Document 1 is configured such that at least a portion of the detection section (the gas passage portion) of the detection element is located on the base end side with respect to the forward end of the metallic shell, it is difficult for the gas-to-be-detected to flow along the detection section (the gas passage portion), and, as a result, the responsiveness of the gas sensor may deteriorate.
Meanwhile, in the gas sensor disclosed in Patent Document 2, a gas introduction space is formed on the forward end side of the interior of the metallic shell. The gas introduction space is concave toward the base end side. By virtue of this configuration, in the gas sensor disclosed in Patent Document 2, the gas-to-be-detected is introduced into the gas introduction space inside the metallic shell through the gas introduction holes of the inner protector, and then flows toward the forward end side. Therefore, in the gas sensor disclosed in Patent Document 2, even when at least a portion of the detection section (the gas passage portion) of the detection element is disposed on the base end side with respect to the forward end of the metallic shell, after being introduced into the gas introduction space inside the metallic shell through the gas introduction holes of the inner protector, the gas-to-be-detected can flow along the detection section (the gas passage portion) of the detection element.
However, in the gas sensor disclosed in Patent Document 2, since the gas-to-be-detected is introduced into the gas introduction space inside the metallic shell after passing through the gas introduction holes of the inner protector, conceivably, the gas-to-be-detected flows turbulently in the gas introduction space. Therefore, the gas-to-be-detected flows turbulently around the forward end portion of the detection element as well, and the gas-to-be-detected may fail to smoothly flow along the detection section (the gas passage portion). Therefore, the responsiveness of the gas sensor disclosed in Patent Document 2 is also susceptible to deterioration.